Most thermochemical treatment techniques for fuel production entail pyrolysis and/or gasification. These processes are traditionally practiced with lignocellulosic biomass and coal. Pyrolysis involves heating the feedstock in the absence of oxygen and leads to cleavage of chemical bonds to yield smaller molecules alkanes, alkenes, alkadienes, carboxylic acids, aromatics, and small amounts of gaseous products. Collectively, the condensable, non-gaseous components are called bio-oil. The pyrolyzed material or original feedstock can also be gasified by heating in the presence of a limited amount of air/steam. Gasification produces syngas typically containing a mixture of CO, H2, CO2 and H2O and CH4. The goal of traditional pyrolysis/gasification methods is to produce bio-oil or syngas and these processes are carried out at elevated temperatures (typically 800 to 950° C.).
A thermochemical pretreatment technology, “torrefaction” has been suggested for lignocellulosic biomass. In torrefaction, the biomass is heated in the absence of oxygen, at 200°-300° C. and near atmospheric pressure. At these relatively low temperatures, thermally labile components of lignocellulosic biomass, such as hemicellulose, are volatilized leaving behind a stable, hydrophobic residue, with higher energy density than the original feedstock. After the torrefaction, the product (solid residue) can be compacted to produce energy-dense pellets. Thus, the primary goal of torrefaction of lignocellulose is to improve feedstock storage and transport. In addition, the torrified residue is directly compatible with traditional pyrolysis and gasification methods and suitable for co-firing with coal.
For lignocellulosic material, hemicelluloses and cellulose display narrow volatilization temperature regions (220-300 and 315-400° C., respectively). However, lignin volatizes over a broad temperature range of 200-900° C. This overlap of volatilization/degradation temperatures between the different lignocellulose polymers prevents separation of distinct biopolymer-specific volatile fractions. Therefore, for lignocellulosic materials, torrefaction is merely a feedstock pretreatment technique to facilitate better storage and transport.
Traditionally bio-oils are upgraded through hydrogenation and deoxygenation to infrastructure-compatible high energy density fuels such as green gasoline or diesel. Generally, bio-oils produced after pyrolysis are separately subjected to these upgradation methods. However, this approach leads to loss of volatiles as well as to polymerization and gumming reactions during storage or transport. Traditionally fuels from algae and other oleaginous materials involve extraction of the triglycerides followed by transesterification with methanol or ethanol.